Heat developable diazo type sheets

ABSTRACT

A heat-developable diazo type sheet containing an acidstabilized diazonium salt in one layer thereof and an acidneutralizing component in another layer thereof is shown wherein at least half of the basic equivalents of the acid-neutralizing component are provided by at least one weakly basic benzimidazole acid-neutralizer compound.

United States Patent i191 Winslow et a1.

[ ]Aug. 28, 1973 HEAT DEVELOPABLE DIAZO TYPE SHEETS [73] Assignee:Minnesota Mining and Manufacturing Company, St. Paul, Minn.

[22] Filed: Nov. 29, 1971 [21] Appl. No.: 203,011

Related U.S. Application Data [63] Continuation of Ser. No. 723,263,April 22, 1968.

[52] U.S. Cl. 96/75, 96/49.91 R, 250/65 T, 117/368, 117/369 [51] int. C1G03C l/60 [58] Field of Search 96/49.75, 91; 250/65 T; 117/368, 36.9

[56] References Cited UNITED STATES PATENTS 2,548,845 4/1951 Neumann96/91 R 2,566,167 8/1951 Carroll et al 96/104 3,199,982 10/1965Kashiwabara..... 96/91 R X 3,248,220 4/1966 Van Rhijn 96/91 R 3,284,20111/1966 Meijs et al. 96/75 3,294,542 12/1966 Sus et a1.... 96/91 R3,294,534 12/1966 Halperia 96/49 3,353,984 ll/1967 Landau 96/75 X3,380,830 4/1968 Schaefi'er 96/75 3,386,828 6/1968 Slimowicz..... 96/91R 3,520,691 7/1970 Scheler 96/75 FOREIGN PATENTS ,OR APPLICATIONS983,665 2/1965 Great Britain 96/91 R 1,032,508 6/1966 Great Britain96/75 OTHER PUBLICATIONS Dietzgen, J. E., Reproductions Review, 2/1962,p. 14, 15, 22 and 26.

Primary Examiner-Charles L. Bowers, Jr. Attorney-Alexander, Sell, Steldt& Delahunt [57] ABSTRACT 8 Claims, No Drawings HEAT DEVELOPABLE DIAZOTYPE SHEETS This is a continuation of application Ser. No. 723,263,filed Apr. 22, 1968.

Heat-developable diazotype sheets commonly employ diazonium salts,acidic compounds for inhibiting premature coupling or self-couplingreactions, and acid-neutralizing components for neutralizing the acidiccompounds, thereby allowing coupling to occur. The acid-neutralizingcompounds commonly are made available for neutralization by theapplication of heat to the diazotype sheets; e.g., heat may causeevolution of acid-neutralizing components, as taught by U. S. Pat. No.2,732,299; or may permit migration of acidneutralizing components intoother components of diazotype sheets by heat-softening of hinder orbarrier materials as taught by British Pat. No. 1,032,508.

Few heat-developable diazotype sheets which exhibit good storageproperties are also capable of development by exposure to reasonabletemperatures for short periods of time. Conversely, diazotype sheetswhich are capable of development by exposure to reasonable temperaturesfor short periods of time commonly exhibit relatively poor storageproperties. Poor storage properties are evidenced by pre-reactionbetween diazonium salts and 'azo-couplei' compounds to form dy'es priorto light exposure or by self-coupling of diazonium salt compounds,causing the development of background fog and reducing contrast betweenthe image and background. These phenomena are frequently attributable tohighly basic acid-neutralizer components which are normally required toovercome the acidstabilization of diazonium salts and to provide rapidcoupling reaction rates. The use of less highly basic componentscommonly necessitates an undesirable increase in the severity ofdevelopment conditions to compensate for reduced coupling reactionrates. The rate of the coupling reaction is highly dependent upon thebasicity of the coupling reaction environment, e.g., highly basicenvironments .favor rapid coupling reaction rates as shown in J. Kosar,Light Sensitive Systems, pages 216-217, John Wiley and Sons, New York,1965. On pages 263-277 of this reference are described variousunsatisfactory attempts which have been made to provide diazotype sheetshaving reasonable storage properties which utilize highly basicacid-neutralizing components to provide rapid coupling reaction rates.Highly basic acid-neutralizing components or precursors of suchcompounds commonly are physically separated from diazonium salts indiazotype sheets to achieve storage properties, but this physicalseparation causes an undesirable reduction in coupling reaction rates.Reduction of the degree of separation between components, however,adversely affects the storage properties of such sheets.

The primary object of this invention is to provide a heat-developablediazotype sheet which exhibits excellent storage properties and iscapable of development by exposure to reasonable temperatures for shortperiods of time.

Another object of this invention is to provide a heatdevelopablediazotype sheet capable of utilizing a great variety of diazonium salts.

Briefly, the present invention relates to a heatreactive diazotype sheethaving at least two layers. One of the layers comprises anacid-stabilized diazonium salt capable of reacting with an azo-couplercompound to form a dye. Another of the layers comprises anacidneutralizer component capable of providing basic equivalents in anamount at least sufficient to render the acid-stabilized diazonium saltreactive with the azocoupler compound. At least one weakly basicbenzimidazole acid-neutralizer compound contributes at least half of thebasic equivalents provided by the acidneutralizer component. Minoramounts of basic ingredients other than weakly basic benzimidazoles mayoften be advantageously incorporated into the diazotype sheet of thepresent invention. Preferably, how ever, such other ingredients provideless than 25 percent of the basic equivalence required to render theacid-stabilized diazonium salt reactive with the azocoupler compound. Inthe most preferred embodiment, a weakly-basic benzimidazole compound isthe sole acid-neutralizer component of the diazotype sheet. Thediazotype sheet of the present invention may additionally containazo-coupler compounds although the azocoupler may be contained in aseparate sheet or may be brought into contact with the diazotype sheet.

It is essential that benzimidazole compounds and diazonium salts be keptin separate layers prior to heatdevelopment of the diazotype sheets ofthe present in vention because of the deleterious effect of benzimidazole compounds upon the storage properties of diazonium salts. Acidiccompounds may often be advantageously incorporated in layers containingdiazonium salts to prevent premature coupling or self-coupling.Diazonium compounds which are acid-salts or to which acid compounds havebeen added are herein referred to as acidstabilized.

Benzimidazole compounds have heretofore been utilized inheat-developable diazotype sheets to enhance dye colors, as taught byBritish Pat. Nos. 967,899 and 1,032,508. Such diazotype sheets, however,have always utilized such strongly basic compounds as ammonia or otherorganic, non-aromatic amines or salts of such amines as acid-neutralizercomponents. Benzimidazole compounds in general are but weakly basic incomparison to such amines and to other basic compounds commonly used indiazo processes. In fact, benzimidazole and many of its derivativesexhibit pseudoacidic characteristics, reflected by their ability to formsalts with metals. It is consequently surprising that benzimidazolecompounds are useful as acidneutralizers without significantlyincreasing the severity of development conditions normally required fordiazotype sheets. Perhaps the greatest advantage realized from theutilization of benzimidazole compounds as the predominant, andpreferably the sole acid-neutralizer components of such diazotype sheetsis the heretofore seldom attainable excellent storage properties nowobtained with the present invention without significant sacrifice incoupling reaction rates during heatdevelopment.

The weakly basic nature of benzimidazole compounds is explained andexemplified in Hofmann, The Chemistry of Heterocyclic Compounds, Part 1,pp. 248-252, lnterscience Pub. N.Y., 1953. The weakly basicbenzimidazole compounds of the present invention are defined asbenzimidazole compounds which are sufficiently basic to neutralize acidsbut which are less basic than imidazole. Such benzimidazole compoundsinclude the following: benzimidazole; 6-

nitrobenzimidazole; 2-chlorobenzimidazole; 2- methylbenzimidazole;Z-ethylbenzimidazole; 2-npropylbenzimidazole; Z-n-butylbenzimidazole;

2-hydroxymethylbenzimidazole; 2-( l '-hydroxyethyl)- benzimidazole;2-(2'-hydroxyethyl)benzimidazole; 2- (l '-hydroxypropyl)benzimidazole;2-(2'- hydroxypropyl benzimidazole; 2-(3'-hydroxypropyl)- benzimidazole;2-(1'-hydroxypentylbenzimidazole; 2-

(l',2'-dihydroxyethyl)benzimidazole; 2- phenylbenzimidazole;2-benzylbenzimidazole; 2- aminomethylbenzimidazole; 2-chloromethylbenzimidazole; 2-

cyanomethylbenzimidazole; zimidazole; 2-meth'yl-5-chlorobenzimidazole;2- methyl-S,6-dimethylbenzimidazole; 2-(1'-hydroxy) ethyl-S,6-dimethylbenzimidazole; 2-( 1 '-hydroxy-lmethyl)ethyl-S,6-dimethylbenzimidazole; 1,2-bis-(benzimidazolyl-2)ethanol; 1,2-bis-(benzimidazolyl- 2)glycol andl,3-bis-(benzimidazolyl-2) propanol-l. The benzimidazole compounds usedin the present invention preferably are non-coupling. The preferredbenzimidazole compounds are benzimidazole, 2- methylbenzimidazole,Z-phenylbenzimidazole and 5,6- dimethylbenzimidazole. Of thesecompounds, benzimidazole is especially preferred.

Although the following explanation should not be construed as binding onapplicant, it is believed that weakly basic benzimidazole compoundscontained within a diazotype sheet tend to readily migrate within thesheet during heat-development, thereby neutralizing reaction-inhibitingacidic compounds and providing a weakly basic environment within whichcoupling of the diazonium salts with azo-coupler compounds may occur.The apparent reactivity of a benzimidazole compound consequently dependsnot only upon its basicity but also upon the ease with which it canmigrate within a diazotype sheet under the influence of heat. Manybenzimidazole and bis-benzimidazole compounds are less preferable in thepresent invention than benzimidazole itself even though they are morestrongly basic than benzimidazole.

it is desirable that the weakly basic benzimidazole compounds beseparated from the diazonium salts in the present invention, sincebenzimidazole compounds generally have an unfavorable influence on thestorage properties of diazonium salt. This unfavorable influence canoften be obviated by including acidic compounds such as tartaric oroxalic acids within the layer containing the benzimidazole compound andthe diazonium salts. Difficulty is then often experienced, however, inpromoting the coupling reaction, e.g., an unusually large amount ofbenzimidazole compound is required. Separation of the benzimidazolecompounds from the diazonium salts may be effected by incorporatingthese compounds in different layers or by introducing a barrier layerbetween layers containing these compounds.

Diazonium salts which find utility in this invention are those which arecapable of coupling with azocoupler compounds in weakly basicenvironments to form dyes. Of the many diazonium salts which may be usedin this invention, the following have given good results:p-amino-N-benzyl-N-ethylbenzenediazonium chlorostannate,p-amino-N-benzyl-N- ethylbenzenediazonium chloro-zincate, 3chloro-4-diethylaminobenzenediazonium chlorozincate,pdiethylaminobenzenediazonium fluoroborate,pdiethylamino-2-ethoxybenzenediazonium chlorozincate,4-diethylamino-2-methylbenzenediazonium chlorozincate,p-morpholinobenzenediazonium fluorobo- 2-methylcarbonylbenrate,p-diazo-N-methyl-N-hydroxyethylaniline zinc chloride,p-diazo-N-ethyl-o-toluidine zinc chloride,pdiazo-N-diethyl-m-phenetidine zinc chloride,pdiazomorpholino-2,5-diethoxybenzene zinc chloride,p-diazomorpholino-2,S-dibutoxybenzene boro fluoride,p-diazo(4-toluyl)mercapto-2,5- dimethoxybenzene zinc chloride,4-diazo-l,2,5- triethoxydiphenyloxalate,p-diazomorpholino-2,5-dibutoxybenzene zinc sulfate, p-diazodimethylaniline 1/l-ZnCl p-diazo diethyl aniline zinc chloride,pdiazo-N-hydroxyethyl-N-ethylanilinel /2 zinc chloride,p-diazo-N-ethyl-N-benzylaniline zinc chloride, 4-diazophenylmorpholinezinc chloride, 1 -diazo-4- benzoylamino-2,S-diethoxybenzene zincchloride. Preferred diazonium salts include: p-diazomorpholino-2,5-dibutoxybenzene borofluoride,p-diazomorpholino-2,5- dibutoxybenzene zincsulfate, 4-diazophenylmorpholine zinc chloride. Light-sensitivediazonium salts are preferably used.

Azo-coupling compounds which find utility in this invention are thosewhich will couple with diazonium salts in weakly basic environments toform dyes. Such couplers include: 2,3-dihydroxy naphthalene, 2,3-dihydroxynaphthalene-6-sulfonic acid sodium salt,2,7-dihydroxynaphthalene, l,7-dihydroxynaphthalene, phloretin,acetoacetanilide, aceto-ace't-o-ansidide, 3-methyl-p-sulfophenyl-S-pyrazolone, aceto-acet-2,5- dimethoxyanilide,aceto-acet-p-toluidide, aceto-acet- 2,4-xylidide, 2,6-dihydroxy toluene,3-methyl-l-phenyl-S-pyrazolone, 2-chloroacetanilide,ethylenediaminebisacetoacetamide, N,N- ethylenebisacetoacetamide,'y-naphtol, B-naphtol,

spectral absorption characteristics. For example, in a.

preferred embodiment, the dyes resulting from the coupling ofp-diazomorpholino 2,5-dibutoxy boro fluoride with acetoacetanilide,2,7-dihydroxy-naphthalene, and 3-hydroxy-Z-naphtho-o-toluidide areyellow, reddishpurple, and cyan, respectively, and together formsubstantially black images.

it is often desirable to incorporate acidic compounds, such as oxalicacid, in diazotype sheets which contain highly-reactive diazonium saltsfor the purpose of inhibiting self-coupling reactions. Diazotype sheetswhich contain less highly-reactive diazonium salts generally requireproportionately reduced amounts of acidic compounds, and many diazoniumacid-salts exhibit sufficient stability without the addition of otheracidic compounds.

A binder material, normally polymeric, is preferably used to contain thereactive components in layers in the diazotype sheets of the presentinvention. The binder materials and barrier materials are preferablythermoplastic and soften sufficiently at reasonable developmenttemperatures to permit benzimidazole compounds to readily migratetherethrough. Thermoplastic binder materials in addition should besufficiently rigid at normal storage temperatures to hinder suchmigration. It is desirable that the binder materials used in the presentinvention be capable of forming continuous films. Examples of suchpolymeric binder materials include polyvinyl chloride acetate,polystyrene, polymethylmethacrylate, etc. Such binder materials arepreferably chosen to be compatible with the components incorporatedtherein, i.e., capable of forming a single phase system with suchcomponents, and permit the ready migration of benzimidazole compoundstherethrough upon heating. However, polymeric binder materials which areincompatible with the components contained therein, such as polymericlatices, may often be advantageously utilized. Non-thermoplastic andnon-continuous binder materials such as paper and cloth may also be usedin this invention to yield desirable results. Binder materials whichpermit the migration therethrough of chemical compounds such asbenzimidazole compounds, couplers, and the like are generally known tothe art, and the selection therefrom is not critical to the presentinvention.

The supporting substrates for the layers of the present invention may beof any suitable material such as plastic film, glass, paper, metal,cloth, wood, etc. Plastic films, such as polyester and cellulosetriacetate, are preferred. The substrates desirably should have aheatdistortion temperature sufficiently high to permit reasonabledevelopment temperatures to be used without damage thereto. It iscontemplated, however, that substrates having lower heat-distortiontemperatures may be desirable, as, for example, when a wrinkled designpattern is desired. It is further contemplated that a separatesupporting layer may be advantageously eliminated in some cases, as whenat least one of the binder materials is a self-supporting film.

The diazotype sheets of the present invention may be prepared by avariety of methods, such as consecutively coating polymer emulsionscontaining reactive components upon a substrate to form layers;laminating together reactive component-containing layers, etc.Preferably, however, the reactive components and polymeric bindermaterial for each layer are combined in a solution which is coated anddried to form a layer. The solvent for each solution is preferably anonsolvent for the binder material of the preceding layer, therebyavoiding the migration which might otherwise occur should a solvent becapable of dissolving the binder material of the preceding layer toliberate the reactive components contained therein.

In the following examples the densities of the lightstruck background(D,,,,,,) and the non-light-struck imaged areas (D,,,,,,) of diazotypesheets of the present invention were measured with a MacBeth QuantalogDensitometer Model OT20 using the green filter (Mac- Beth Corporation).

The following examples are presented for illustrative purposes only andshould not be construed as limiting the spirit or scope of the presentinvention.

EXAMPLE 1 A solution of:

25 g. polyvinyl chloride acetate copolymer (Bakelite VYHH, Union CarbideCorp.)

2.2 g. benzimidazole 53 g. acetone 22 g. methyl ethyl ketone wasprepared and was coated onto the surface of polyester film. After dryingfor 2 minutes at 96 C., a layer having a dry coating weight of 15.1 g./mwas obtained.

Over this dry layer was coated a solution of:

10 g. alcohol-soluble cellulose acetate butyrate polymer (Eastman EASB)1.0 g. 3-hydroxy-2-naphtho-o-phenetidide 1.4 g.p-diazomorpholino-Z,S-dibutoxybenzene boro 10 g. butanol g. methanolUpon drying for 2 minutes at 49 C., a layer of 5.38 g./m coat-ing weightwas obtained.

The diazotype sheet thus prepared was exposed for 16 seconds through atest negative having a density gradient ranging from D,,,,,, of 0. 10 toD of 2.16 to radiation from a 250 watt mercury-vapor lamp positioned3.18 cm. from the recording element. The exposed sheet was developed byexposure for 30 seconds to a temperature of 121 C. A bluish-purple imageexhibiting a D of 1.20 was obtained. The D of the lightstruck backgroundareas was 0.06.

EXAMLE 2 Upon the surface of a polyester film was coated a solution of:

25 g. polyvinyl chloride acetate copolymer (Bakelite VYHH, Union CarbideCorp.) 2.2 g. benzimidazole 0.6 g. aceto-acet-o-anisidide A coatingweight of 15.1 g./m was obtained upon drying this layer for 2 minutes at96 C.

Upon this dry layer was coated the following solution:

10 g. alcohol-soluble cellulose acetate butyrate polymer (Eastman EASB)2.0 g. p-diazomorpholino-2,S-dibutoxybenzene boro fluoride 0.80 g.4-diazophenylmorpholine zinc chloride 0.80 g.3-hydroxy-2-naphtho-o-phenetidide 0.80 g.3-hydroxy-2-naphtho-o-toluidide 0.12 g. 2,3-dihydroxy naphthalene 0.40g. oxalic acid 10.0 g. butanol 80.0 g. methanol. After drying for 2minutes at 49 C., a dry layer having a coating weight of 15.1 g./m wasobtained. 4

The diazotype sheet thus prepared was exposed and developed as inExample 1. The resulting reddish-black image exhibited a D of 1.70. TheD of the lightstruck background was 0.06. A sample of the unexposed wasaged for 6 months at 23 C., 50 percent R.H., and was then exposed anddeveloped as above, yielding a D, value of 0.11 and a D of 1.30.

EXAMPLE 3 A solution of: 25 g. polyvinyl chloride acetate copolymer(Bakelite VYHH, Union Carbide Corp.)

2.2 g. benzimidazole 0.60 g. aceto-acet-2,4-xylidide 0.80 g.3-hydroxy-2-naphtho-o-phenetidide 53 g. acetone 22 g. methyl ethylketone was coated upon the surface of 80 lb SP-l paper (Lee ShoellerCompany) and was dried for 2 minutes at 96 C. to yield a dry layerhaving a coating weight of 15.1 g./m

Upon this dry layer was coated a solution of g. alcohol-solublecellulose acetate butyrate polymer (Eastman EASB) 2.0 g.p-diethylaminobenzene diazonium fluoroborate 0.60 g. oxalic acid which,upon drying for 2 minutes at 49 C., yielded a dry layer having a coatingweight of 5.38 g./m

The diazotype sheet thus prepared was exposed and developed as inExample 1. A green image was obtained having a D of 1.30. The backgroundexhibited a D of 0.14.

EXAMPLE 4 A solution of: 10 g. polyamide resin (Elvanol 8061, E. l.DuPont de Nemours) 2.2 g. benzimidazole 0.60 g. aceto-acet-p-toluidide10 g. butanol 80 g. methanol was coated on the surface of polyesterfilm. The coating weight of this layer after drying for 2 minutes at 96C. was 6.46 g./m

Upon this dry layer was coated a solution of:

5.0 g. cellulose acetate (N-200, Dow Chem. Co.)

2.0 g. p-diazomorpholino-2,5-dibutoxy-benzene boro 0.80 g.4-diazophenylmorpholine zinc chloride 0.80 g.3-hydroxy-2-naphtho-ophenetidide 0.80 g. 3-hydroxy-2-naphtho-o-toluidide0.12 g. 2,3-dihydroxynaphthaline 0.40 g. oxalic acid 10 g. methyl ethylketone 85 g. acetone After drying for 2 minutes at 49 C., the coatingweight of this layer was found to be 5.38 g./m

The diazotype sheet thus prepared was exposed for 16 seconds through asuitable negative to radiation from a 140 ampere carbon are positioned0.915 m from the element. The sheet was then developed as in Example l,yielding a reddish-black image having a D of 1.65. The D of thebackground was 0.07.

EXAMPLE 5 A solution of:

Upon exposure and development as in Example 4, a cyan image was obtainedwhich exhibited a B of 1.00. The D of the background was 0.08.

EXAMPLE 6 Upon the surface of polyester film was coated a solution of:

25 g. polyvinyl chloride acetate copolymer (Bakelite VYHl-l, UnionCarbide Corp.)

2.2 g. benzimidazole 53 g. acetone 22 g. methyl ethyl ketone whichyielded a layer having a coating weight of 15.1 g./m upon drying for 2minutes at 96 C.

Over this dry layer was coated a solution of:

20 g. polystyrene (Dow Chemical Co. No. 700) 40 g. toluene 40 g. acetoneto form, after drying for 2 minutes at 96 C., a second layer having acoating weight of 5.38 g./m

Upon the dry second layer was coated a solution of:

10 g. alcohol-soluble cellulose acetate butyrate polymer (Eastman EASB)2.4 g. p-diazomorpholino-2,5-dibutoxybenzene boro 1.0 g.2,3-dihydroxynaphthalene 1.0 g. oxalic acid to form, after drying for 2minutes at 49 C., a third layer having a coating weight of 5.38 g./m

The diazotype sheet thus produced was exposed and developed as inExample 1, yielding a bluish-purple image having a D of 0.07. Thebackground exhibited a D of 1.30. A sample of the unexposed diazotypesheet was aged for 6 months at 23 C., 50 percent R.l-l., and then wasexposed and developed as above. D and D,,,,, values of 0.15 and 1.00respectively were obtained.

EXAMPLE 7 A solution of:

25 g. polyvinyl chloride acetate (Bakelite VYHH,

Union Carbide Corp.)

2.2 g. benzimidazole 53 g. acetone 22 g. methyl ethyl ketone was coatedonthe surface of 80 lb. SP-l paper (Lee 10 g. polyvinyl alcohol(Gelvatol, Shawinigan Chem. 50

Shoeller,,Company). After drying for 2 minutes at 96 C., the coatingweight of this layer was 15.1 g./m

Upon this dry layer was coated a solution of: 20 g.polystyrene-acrylonitrile copolymer (Tyril 770,

Dow Chemical Co.) 0.60 g. aceto-acet-o-anisidide g. acetone to form asecond layer having a coating weight of 7.55 g./m after drying for 2minutes at 96 C.

Upon the dry second layer thus formed was coated a solution of:

5 g. alcohol-soluble cellulose acetate butyrate polymer (Eastman EASB) 5g. polyvinyl acetate (Gelva V-l00, Dow Chem.

Co.) 2.4 g. p-diazomorpholino-2,S-dibutoxybenzene boro fluoride 0.12 g.2,3-dihydroxy naphthalene 0.08 g. 2,7-dihydroxy naphthalene 0.80 g.3-hydroxy 2-naphtho-o-phenetidide 0.80 g.3-hydroxy-Z-naphtho-o-toluidide 0.40 g. oxalic acid 10 g. butanol 80 g.methanol to form a third layer which, after drying for 2 minutes at 49C., exhibited a coating weight of 5.38 g./m

The diazotype sheet thus obtained was exposed and developed according toExample 1, yielding a bluishblack image having a D of 1.20. The 1),, ofthe non-imaged areas was 0.15.

EXAMPLE 8 A sheet of paper (80 lb. SP-l, Lee Schoeller Company) wassaturated with a solution of:

3 g. 3-hydroxy-2-naphtho-o-phenetidide 3 g.p-diazomorpholino-2,S-dibutoxybenzene 0.4 g. oxalic acid 94 g. methanolA coating weight of 2.25 g./m was obtained upon drying for 2 minutes at96 C. The coated paper was imaged as in Example 1 and was divided intoidentical portions A, B and C.

A second paper substrate (80 lb. SP-l Lee Schoeller Company) was dividedinto two portions X and Y which were treated as follows:

Portion X was saturated with a solution of 4.0 grams of benzimidazole in96 ml. of methanol. Upon drying for 2 minutes at 49 C., a coating weightof 1.65 grams (0.0014 g. moles) of benzimidazole per square meter wasobtained.

Portion Y was saturated with a solution of 2.61 grams of ammoniumacetate in 97.4 ml. of methanol to give a coating weight of 0.065 gram(0.00845 g. moles) of ammonium acetate per square meter upon drying for2 minutes at 49 C.

lmaged portions A, B and C were developed as follows:

Portion A was placed in facing contact with portion X (containingbenzimidazole) and was heated at 121 C. for 10 minutes. Similarly,portion B was placed in facing contact with portion Y (containingammonium acetate) and was heated at 121 C. for 10 minutes. Portion C wasexposed to a saturated atmosphere of ammonia at 60 C. for 10 minutes ina commercial diazo developing unit (Uniprinter 086, Model No. 49BG,Minnesota Mining and Manufacturing Co.).

The intensities (D,,,,,,) of the images formed on portions A, B and Cwere measured as a function of time, yielding the following values:

seconds seconds seconds Portion A, D 0.66 0.74 0.85 Portion B, D. 0.600.72 0.74 Portion C, D 0.79 0.84 0.87

These data show the surprising efficiency of benzimidazole as anacid-neutralizing compound in comparison to the efficiency of the highlybasic, often-used acidneutralizer, ammonia.

EXAMPLE 9 A solution of:

25 g. polyvinyl chloride acetate copolymer (Bakelite VYHH, Union CarbideCorp.)

2.5 g. Z-aminobenzimidazole 53 g. acetone 22 g. methyl ethyl ketone 5 10g. alcohol-soluble cellulose acetate butyrate polymer (Eastman EASB) 1.2g. 2,3-dihydroxynaphthalene 2.0 g. p-diazomorpholino-2,5-dibutoxybenzeneboro fluoride 10 0.2 g. oxalic acid 80 g. methanol 10 g. butanol toyield a layer which exhibited a coating weight of 5.38 g./m after dryingfor 2 minutes at 49 C.

Upon exposure and heat-development as in Example 1, a light-blue imageupon a clear background was obtained.

EXAMPLES 10-13 Example 9 was repeated, except that the 2-aminobenzimidazole was replaced by 2.5 g. of the following benzimidazolecompounds:

Example 10: Z-methylbenzimidazole Example 11: 2-phenylbenzimidazoleExample 12: 5,6-dimethylbenzimidazole Example 13: Z-hydroxybenzimidazoleIn each case, a light-blue image upon a clear background was obtained.

Various other embodiments of the present invention will be apparent tothose skilled in the art without departing from the scope thereof.

What is claimed is:

l. A heat reactive diazotype sheet having at least two layers, one ofsaid layers comprising a heat-softenable component capable of providingbasic equivalents in an amount at least sufficient to neutralize saidacid stabilization of said diazonium salt, to provide a weakly basicenvironment for the diazonium salt in the sheet, and to render saidacid-stabilized diazonium salt reactive with said azo-coupler compound,at least one of said layers containing an azo-coupler, and greater thanpercent of said basic equivalents being provided by at least oneweakly-basic benzimidazole acid-neutralizer compound.

2. A heat reactive diazotype sheet having at least three layers, one ofsaid layers comprising a heatsoftenable binder material and anacid-stabilized diazonium salt capable of reacting with an azo-couplercompound to form a dye, another of said layers comprising 5 aheat-softenable binder material and an acidneutralizer component capableof providing basic equivalents in an amount at least sufficient toneutralize said acid stabilization of said diazonium salt to render saiddiazonium salt reactive with said azo-coupler compound by providing aweakly basic environment for the diazonium salt in the sheet, greaterthan 75% of said basic equivalents being provided by benzimidazole, atleast one of said heat softenable binder layers additionally includingan azo-coupler compound, and between said diazonium salt containinglayer and said benzimidazole-containing layer a barrier layer,permeable, upon heating, to benzimidazole.

3. A light-sensitive, storage-stable diazotype sheet capable of rapidheat development, said sheet comprising a support bearing at least twolayers, one of said layers comprising a heat-softenable polymeric binderand a dye-forming composition including an azo-coupler compound and alight-sensitive acid-stabilized diazonium salt capable of reacting withsaid coupler compound to form a dye, and another of said layerscomprising a heat-softenable polymeric binder and an acid neutralizercapable of neutralizing said acid stabilization of said acid-stabilizeddiazonium salt in sufficient quantity to provide a weakly basicenvironment for the diazonium salt in the sheet, and to render saiddiazonium salt reactive with said coupler compound, greater than 75percent of the basic equivalents of said acid neutralizer being providedby benzimidazole.

4. The sheet of claim 3 wherein said dye-forming composition comprisesp-diazomorpholino-2,5- dibutoxy benzene borofluoride, acetoacetanilide,2,7-dihydroxynaphthalene and 3-hydroxy-2-naphthoo-toluidine.

5. A light-sensitive, storage-stable diazotype sheet capable of rapidheat development comprising a support bearing at least two layers, oneof said layers comprising a first heat-softenable polymeric bindercontaining a dye-forming composition including an azo coupler compoundand a light-sensitive acid-stablized diazonium salt capable of reactingtherewith to form a dye, and another of said layers comprising a secondheatsoftenable polymeric binder which contains, as the sole acidneutralizer component of said sheet, a sufficient quantity ofbenzimidazole to neutralize said acid stabilization of said diazoniumsalt and to render said diazonium salt reactive with said azo couplercompound, by providing a weakly basic environment for the diazonium saltin the sheet.

6. The sheet of claim 5 wherein said second heatsoftenable polymericbinder is soluble in a non-solvent for said first polymeric binder.

7. A heat-reactive diazotype sheet having at least two layers, one ofsaid layers comprising a heat-softenable binder material and anacid-stabilized diazonium salt capable of reacting with an azo-couplercompound to form a dye, and another of said layers comprising aheat-softenable binder material and an acid-neutralizer componentcapable of providing basic equivalents in an amount at least sufficientto neutralize said acid stabilization of said diazonium salt and torender said diazonium salt reactive with said azo-coupler compound byproviding a weakly basic environment for the diazonium salt in thesheet, greater than of said basic equivalents being provided bybenzimidazole, at least one of said layers additionally including anazo-coupler compound.

8. Thesheet of claim 7 additionally comprising, in said layer containingsaid diazonium salt, an acid stabilizing compound.

2. A heat reactive diazotype sheet having at least three layers, one ofsaid layers comprising a heat-softenable binder material and anacid-stabilized diazonium salt capable of reacting with an azo-couplercompound to form a dye, another of said layers comprising aheat-softenable binder material and an acid-neutralizer componentcapable of providing basic equivalents in an amount at least sufficientto neutralize said acid stabilization of said diazonium salt to rendersaid diazonium salt reactive with said azo-coupler compound by providinga weakly basic environment for the diazonium salt in the sheet, greaterthan 75% of said basic equivalents being provided by benzimidazole, atleast one of said heat softenable binder layers additionally includingan azo-coupler compound, and between said diazonium salt containinglayer and said benzimidazole-containing layer a barrier layer,permeable, upon heating, to benzimidazole.
 3. A light-sensitive,storage-stable diazotype sheet capable of rapid heat development, saidsheet comprising a support bearing at least two layers, one of saidlayers comprising a heat-softenable polymeric binder and a dye-formingcomposition including an azo-coupler compound and a light-sensitiveacid-stabilized diazonium salt capable of reacting with said couplercompound to form a dye, and another of said layers comprising aheat-softenable polymeric binder and an acid neutralizer capable ofneutralizing said acid stabilization of said acid-stabilized diazoniumsalt in sufficient quantity to provide a weakly basic environment forthe diazonium salt in the sheet, and to render said dia-zonium saltreactive with said coupler compound, greater than 75 percent of thebasic equivalents of said acid neutralizer being provided bybenzimidazole.
 4. The sheet of claim 3 wherein said dye-formingcomposition comprises p-diazomorpholino-2,5-dibutoxy benzeneborofluoride, acetoacetanilide, 2,7-dihydroxynaphthalene and3-hydroxy-2-naphtho-o-toluidine.
 5. A light-sensitive, storage-stablediazotype sheet capable of rapid heat development comprising a supportbearing at least two layers, one of said layers comprising a firstheat-softenable polymeric binder containing a dye-forming compositionincluding an azo coupler compound and a light-sensitive acid-stablizeddiazonium salt capable of reacting therewith to form a dye, and anotherof said layers comprising a second heat-softenable polymeric binderwhich contains, as the sole acid neutralizer component of said sheet, asufficient quantity of benzimidazole to neutralize said acidstabilization of said diazonium salt and to render said diazonium saltreactive with said azo coupler compound, by providing a weakly basicenvironment for the diazonium salt in the sheet.
 6. The sheet of claim 5wherein said second heat-softenable polymeric binder is soluble in anon-solvent for said first polymeric binder.
 7. A heat-reactivediazotype sheet having at least two layers, one of said layerscomprising a heat-softenable binder material and an acid-stabilizeddiazonium salt capable of reacting with an azo-coupler compound to forma dye, and another of said layers comprising a heat-softenable bindermaterial and an acid-neutralizer component capable of providing basicequivalents in an amount at least sufficient to neutralize said acidstabilization of said diazonium salt and to render said diazonium saltreactive with said azo-coupler compound by providing a weakly basicenvironment for the diazonium salt in the sheet, greater than 75% ofsaid basic equivalents being provided by benzimidazole, at least one ofsaid layers additionally including an azo-coupler compound.
 8. The sheetof claim 7 additionally comprising, in said layer containing saiddiazonium salt, an acid stabilizing compound.